1. Electronic PRINCIPLES
MALVINO & BATES
Electronic
PRINCIPLES
SEVENTH EDITION

2. Chapter 10
Voltage Amplifiers

3. Topics Covered in Chapter 10
Voltage gain
The loading effect of input impedance
Multistage amplifiers
Swamped amplifier
Two-stage feedback
Troubleshooting

4. Voltage gain Ac output voltage divided by ac input voltage
Can be derived by using the p model of a transistor
Ac collector resistance divided by the ac resistance of the emitter diode

5. A VDB common-emitter amplifier
+VCC RE R2 RC R1 RL
vin
vout

6. p model of the common-emitter amplifier
This model best illustrates that
zin(stage) = R1 R2 bre’ ib
vin
vout R1 R2
bre’ ic RC RL

7. T model of the common-emitter amplifier
AV = rc
re’
This model best illustrates that RC RL
vout ic ie R1 R2
re’
rc = RC RL
vin ie
vout
AV =
vin

8. Calculating voltage gain
Solve the dc circuit to find the emitter current.
Use the emitter current to find re’.
Combine RC and RL to find rc.
Divide rc by re’.

9. Loading effect of input impedance
An ideal ac voltage source has zero source resistance
The input impedance of a stage includes biasing resistors and base input impedance
When the ac source is not stiff, the input voltage is less than the source voltage

10. Real signal sources are not ideal.Rg
vin vg
zin(stage)
When a source is not stiff, use:
vin =
zin(stage)
+ zin(stage) Rg vg

11. Multistage amplifier Two or more amplifiers cascaded
Provides increased gain
Two CE stages produce an amplified in-phase signal

12. To get more gain, a cascade amplifier can be used.
Stage 1
AV1
Stage 2
AV2 RL
vout
vin
zin(stage 2)
The overall voltage gain: AV = AV1AV2

13. Ac equivalent circuit for the two-stage amplifierRg
zin(stage 2) RC RL
zin(stage 1) ic RC ic vg
The 2nd stage loads the 1st stage:
Rc1 = Rc1 ║ Zin (stage 2)

14. Swamped amplifier
Some of the emitter resistance is unbypassed to get negative feedback
Voltage gain is stabilized
Input impedance is increased
Large-signal distortion is decreased

15. Swamped amplifier circuit
+VCC
Swamped
amplifier
circuit RC R1
vout RL
vin rE R2
ac feedback resistor RE

16. T model of the swamped common-emitter amplifier
Emitter feedback
decreases the gain
and increases the
impedance. RC RL
vout ic
vin
AV = rc
re’ + re R1 R2
re’
zin(base) = b(re’ + re) re

17. Large signals produce distortion since re’ is non-linear.
vbe ie IE Q
VBE
Input signal

18. This resistor is linearRC RL
vout ic
vin R1 R2
re’
This resistor is linear
and can swamp re’. re

19. vb versus ie in a swamped amplifier
re’ + re Q vb
Input signal

20. Emitter ac feedback
A swamped amplifier is an example of single-stage feedback
Decreases voltage gain (but the gain is more stable)
Increases the base input impedance
Decreases large-signal distortion

21. Two-stage feedback Stage 1 Stage 2 RL vout vin rf
The feedback signal can be connected to
the emitter end of resistor re in stage 1.

22. +VCC
Stage 1 RC R1
To stage 2
Feedback
from stage 2
vin rE R2 RE

23. Voltage gain with two-stage feedback:RL
vout
vin rf
AV = rf
re(stage 1)
+ 1

24. Troubleshooting
Open coupling or bypass capacitors will not cause dc error.
Shorted coupling or bypass capacitors will cause dc error.
An open coupling capacitor will cause a loss of signal.
An open emitter bypass capacitor will cause the gain to decrease.

25. More troubleshooting The supply line is normally an ac ground.
The supply line will not be an ac ground if a bypass or filter capacitor opens.
An open bypass may allow other signals to affect the amplifier.
An open filter capacitor will allow power supply ripple to get into the amplifier.